Toronto — The unlikely combination of freshwater fish and cannabis is producing outsized medical marijuana crops that Green Relief aims to capitalise on, as the Canadian company plots a stock market listing and global expansion. In an underground southern Ontario facility surrounded by farmland, Green Relief operates a cutting-edge aquaponic farm, using filtered fish waste to fertilise cannabis plants, which in turn clean the water for the fish. The company says it is the world’s only licensed producer to grow medical marijuana this way, a pesticide-free process that took two-and-a-half years to fine tune. The only signs of this operation, which is built into a hill and insulated by about three feet of dirt and grass, is above-ground ventilation equipment sticking out of the ground.

“This is the agriculture of the future,” said Warren Bravo, a former concrete contractor who co-founded the company with friend Steve LeBlanc in 2013. “If you’re not latching on to sustainable agri...

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